When a laboratory reports a non-negative drug-test result, that result does not go straight to the employer. It goes first to a Medical Review Officer (MRO) — a licensed physician trained to interpret drug-test results and check for legitimate medical explanations. The MRO is the quality-control and fairness safeguard that separates a defensible program from a liability. Here is what they do and why it matters.

What an MRO is

An MRO is a licensed physician (MD or DO) with knowledge of substance use disorders and the requisite training and certification to review and interpret drug-test results. In DOT-regulated programs the role is mandated by 49 CFR Part 40, and the DOT ODAPC details their responsibilities. The SAMHSA Drug-Free Workplace program describes the same safeguard for federal programs.

What the MRO actually does

Reviews the lab result and chain of custody for any procedural problems that could invalidate the test.
Contacts the employee on a verified non-negative to ask about prescriptions or medical explanations — confidentially.
Evaluates legitimate medical explanations, such as a valid prescription that accounts for the finding.
Verifies the result as positive, negative, or cancelled, and only then reports a verified result to the employer.
Protects private medical information, sharing only the verified result — not the underlying diagnosis.

Why positive results get reviewed

A laboratory can accurately detect a substance, but it cannot know whether that substance is there for a lawful reason. Someone taking a prescribed opioid after surgery, or an amphetamine-based ADHD medication, may screen positive legitimately. Without MRO review, an employer might penalize a person for lawful medical care — a discrimination and ADA risk. The MRO also catches invalid or adulterated specimens flagged by the lab; see how integrity is protected in chain of custody.

MRO review vs. confirmation testing

These are two different safeguards, and you want both. LC-MS/MS confirmation answers "is the molecule really there, and how much?" The MRO answers "is there a legitimate medical reason?" A screen alone, without confirmation and MRO review, is not a defensible basis for action — see lab-confirmed vs instant strips. Together they make a result stand up to scrutiny, whether in pre-employment testing or a reasonable-suspicion event.

When is an MRO required?

MRO review is mandatory for all DOT-regulated testing. For non-DOT programs it is a best practice that many employers adopt precisely because it reduces legal exposure and improves fairness. If your program acts on positives, an MRO belongs in the workflow — see our 49 CFR Part 40 guide.

The MRO interview: what to expect

When a laboratory reports a non-negative, the MRO — not the employer — contacts the donor first, confidentially. The conversation is narrow and factual: the MRO asks whether there is a legitimate medical explanation, such as a valid prescription, that accounts for the specific substance found. The donor may be asked to provide verification (for example, pharmacy or prescriber confirmation). This step exists precisely because a laboratory can confirm what is present but cannot know why. It protects employees on legitimate medication and shields employers from acting on a lawful prescription. The role and its confidentiality obligations are defined in 49 CFR Part 40 and summarized by DOT ODAPC.

Verified negative, positive, or cancelled

After review, the MRO issues one of three outcomes. A result is verified negative when a legitimate medical explanation accounts for the finding, verified positive when it does not, and cancelled when a fatal procedural flaw — such as a broken chain of custody or an invalid specimen — makes the result unusable. Only the verified outcome, not the underlying medical detail, is reported to the employer. That clean separation is what lets a company act on a result while keeping protected health information private. It also underscores why a bare instant-strip reading, with no MRO in the loop, is not a defensible basis for an employment decision — see lab-confirmed vs instant strips and the fairness rationale in the pre-employment guide.

MRO FAQs

Is an MRO required for non-DOT testing?

It is mandatory for DOT programs and a strong best practice for non-DOT employers, because it reduces legal exposure and protects employees on lawful medication.

Does the employer see my medical information?

No. The MRO reports only a verified result — negative, positive, or cancelled — not your diagnosis or prescriptions.

What if the chain of custody was flawed?

The MRO can cancel the result — see chain of custody.

LabX coordinates MRO review for you

LabX Diagnostic Systems — CLIA-certified (#10D2067145), COLA-accredited in Tampa, Florida — pairs LC-MS/MS confirmation with MRO coordination so employers receive only verified, defensible results. Build your program through employer drug testing, review the test menu, or order kits. Existing clients can sign in to view verified results.

Medical Review Officer (MRO): What They Do